RECONSTRUCTION OF ALLOCHTHONOUS SALT EMPLACEMENT FROM 3-D SEISMIC-REFLECTION DATA, NORTHERN GULF-OF-MEXICO

Analysis of a 110 mi2 (284 km2) three-dimensional (3-D) seismic survey located at the northern Gulf of Mexico continental shelf edge documents the emplacement history of an allochthonous salt structure. A listric, counter-regional growth fault bounds the updip salt edge and apparently soles into a detachment surface from which salt was evacuated. The counter-regional fault system served as the feeder stock for the dome. The current salt shape is an asymmetric, elongated wedge, or one-sided piercement structure with a diameter of 20,000 ft (6.1 km) and maximum thickness of 13,000 ft (4.0 km). A steep fault/salt contact bounded by small, oval-shaped withdrawal subbasins on the landward side contrasts with a shallow-dipping basinward salt/sediment contact. A relatively thin sediment cover onlaps the shallow basinward salt flank. Schematic growth history reconstructions along closely spaced cross sections from the 3-D survey suggest a three-stage emplacement process in which regional extension played a significant role. Regional seismic lines support the presence of a detachment surface in the area apparently separating Upper Cretaceous to lower Tertiary(?) sediments from upper Miocene (7.0-5.4 Ma) and younger sediments. During the first stage, an allochthonous salt sheet was emplaced during the hiatus and then segmented. From the middle to late Pliocene (4.0-2.2 Ma), the salt structure evolved by downbuilding (the second stage), remaining close to the ocean floor surface and covered by only a thin sediment veneer. After the beginning of the Illinoian glacial stage (0.85 Ma), the deep source salt layer was depleted and the dome was quickly buried by up to 1700 ft (520 m) of sediment during the third stage. Seismic amplitude anomalies associated with structural disconformities indicate a possible salt evacuation surface, or salt weld, along the deep feeder stock. Geohistory diagrams indicate extremely rapid local subsidence in the withdrawal basin, which contains up to 12,000 ft (3.6 km) of Pleistocene sediment. Such accelerated subsidence, in conjunction with the fault/salt geometry, contributed to the development of hydrocarbon reservoirs near the dome flanks.